Decreasing prevalence of isolates with vancomycin heteroresistance and vancomycin minimum inhibitory concentrations ≥2 mg/L in methicillin-resistant Staphylococcus aureus over 11 years: potential impact of vancomycin treatment guidelines

• The prevalence of hVISA and isolates with vancomycin MIC ≥2 mg/L is decreasing overtime.
• These changes temporally coincided with increasing vancomycin trough levels.
• Compliance with vancomycin treatment guidelines may have contributed to these changes.

We evaluated vancomycin MIC (V-MIC) and the prevalence of intermediately susceptible (VISA) and heteroresistant (hVISA) isolates trends in methicillin-resistant Staphylococcus aureus bacteremia among 720 adults (≥18 years) inpatients over 4 study periods (2002–2003, 2005–2006, 2008–2009, and 2010–2012). V-MIC (Etest) and the prevalence of hVISA and VISA (determined by population analysis profile–area under the curve) were stratified according to the study period. Mean vancomycin MIC was 1.78 ± 0.39, 1.81 ± 0.47, 1.68 ± 0.26, and 1.54 ± 0.28 mg/L in 2002–2003, 2005–2006, 2008–2009, and 2010–2012, respectively (P < 0.0001). We noted a steadily decreasing prevalence of isolates with V-MIC ≥2 mg/L (50.0%, 45.2%, 35.4%, and 18.7%; P < 0.0001) and hVISA (9.7%, 6.6%, 3.0%, and 2.1%; P = 0.0003). VISA prevalence remained low (0–2%). These changes coincided with steadily increasing vancomycin trough levels (9.9 ± 7.8, 11.1 ± 8.4, 16.6 ± 7.8, and 19.7 ± 5.9 mg/L in 2002–2003, 2005–2006, 2008–2009, and 2010–2012, respectively; P < 0.0001). These changes imply that adherence to vancomycin treatment guidelines may suppress the development of less susceptible isolates.